Organic semiconductors are the subject of intense research and development driven by their potential to enable low cost, flexible electronic devices. They have been used extensively in organic field effect transistors and in circuits integrating multiple devices. OTFTs are expected to become a dominant technology in the display technology sector where their low cost manufacture, flexibility and low weight could result in OTFTs replacing their silicon based counterparts in some cases. However significant improvements are needed in the area of improved organic semiconductor material performance and in device manufacturing.
OTFT devices require organic semiconducting materials that satisfy a challenging combination of performance attributes including: high charge carrier mobility, coupled with high electrical, mechanical and thermal stability. Whilst a wide range of high charge carrier mobility materials are available these tend to be non-polymeric organic molecules that after processing result in brittle layers that limit the flexibility of the device (refer to N. Madhavan, Organic Chemistry Seminar Abstracts 2001-2002 Semester II, University of Illinois at Urbana Champaign, 2002, pp 49-56 at http://www.scs.uiuc.edu/chemgradprogram/chem/435/s02-Madhavan.pdf).
On the other hand, a wide range of amorphous and crystalline semiconducting polymers are available having excellent flexibility and toughness; however these have unfavourably low charge carrier mobilities (L L Chua et al., Nature, March, 2005, Vol. 434, pp 194-198). It has been proposed to use polyacenes, especially substituted, soluble pentacene molecules as semiconductors. Such compounds and a number of electronic devices using them have been previously disclosed in US patent application 2003/0116755, EP 1729357 and U.S. Pat. No. 6,690,029.
It is desirable to improve the stability and integrity of the semiconductor layers and it has been disclosed in WO 2005/055248 to achieve this by using non-polymeric soluble pentacene semiconducting molecules in conjunction with a polymer binder having a permittivity εr, of at most 3.3. WO 2005/055248 reports in tests on top gate transistors that two compositions showed no charge mobility, five compositions showed charge mobility above 1.0 cm2/Vs and nine compositions had mobilities ranging between 0 to 1.0 cm2/Vs. The best result reported was a mobility of 1.44 with a standard deviation of 0.35. Furthermore all 22 examples in the application were in a top gate OTFT configuration. We have found in the case of bottom gate organic thin film transistors that the charge carrier mobility is substantially reduced; WO 2005/055248 leaves the problem of bottom gate transistors unsolved. In WO 2007/078993 an attempt was made to use pentacene semiconducting molecules with binders of permittivity more than 3.3; the best charge mobilities reported were 2×10−2 cm2/Vs. This would appear to reinforce the prior teaching that the binder should have a permittivity below 3.3.
The simplest way of depositing a semiconductor layer, for example a polyacene semiconductor/polymer binder formulation, is to dissolve the semiconductor and polymer in a suitable solvent, deposit the formulated material and evaporate the solvent. Substituted soluble pentacenes have been described in the above specifications and suitable binders for them have been disclosed in WO2005/055248. A number of useful binders are disclosed in WO2005/055248A, WO2007/078993 and US 2007/0158646.
An important aspect of thin film transistors is that their performance in an array should be as uniform as possible. If there is a wide variation in the performance of the transistors when they are manufactured this would result in variability in the end use device performance. For example, if a non-uniform transistor backplane was used in a display application then the pixels in the display would be non-uniform and of unacceptable quality. The problem of non-uniformity of performance increases often very considerably as the dimensions of the transistors are reduced as this involves a reduction in the channel length between the source and drain electrodes. Reduction of the dimensions of electronic components has been a consistent and enduring characteristic of advances in electronics for many years and the problem is of increasing severity.
It is an object of the present invention to provide semiconductor compositions having a combination of a generally good uniformity of performance in electronic devices whilst having good charge mobility.